1. Field of the Invention
The present invention concerns a method of synchronising base stations in a mobile radio telecommunication system. More particularly, the present invention concerns a method of synchronising base stations for a telecommunication system of the time division duplex (TDD) type. The said telecommunication system is for example the system for which a standard is at present being drawn up, normally referred to as 3GPP W-CDMA TDD.
2. Discussion of the Background
FIG. 1 depicts a radio frame of such a telecommunication system. It consists of fifteen time slots, some of which, for example the slots IT0, IT1, IT2, IT5, IT6 and IT8, are intended for conveying data (in the broad sense of the term) in the downlink direction (base station to mobile terminal) whilst others, the slots IT3, IT4, IT7, IT9, IT10, IT11, IT12, IT13 and IT14, are intended for conveying data in the uplink direction (mobile station to base station). During a transmission slot, the data (D) are transmitted in the form of a sequence of symbols. The slot also includes a midamble (M) comprising pilot symbols enabling the channel to be estimated, a power control word (TPC) and a guard period (GP′). In such a system, several mobile terminals or base stations can transmit or receive data in the same time slot. The connections are differentiated by code division multiplexing (Code Division Multiple Access=CDMA). The symbols transmitted by or for the different users are spectrally spread, approximately at a “chip” frequency 1/Tc where Tc is the elementary transmission period.
Because the same frequency can be used both in the uplink direction and in the downlink direction, it is essential to ensure synchronisation of the base stations. This is because, if such were not the case, a first mobile terminal transmitting at high power in an uplink channel could interfere with a second mobile channel, close to the first, receiving data over a downlink channel. The synchronisation constraint between adjacent base stations is around a few microseconds (approximately 5) in the W-CDMA TDD system.
To effect synchronisation between base stations, several methods have been proposed in the state of the art. According to a first method, the synchronisation is achieved by virtue of GPS receivers equipping the base stations. According to a second method, first of all, in an initial phase, for example during the phase of setting up the network or a new base station, an approximate synchronisation is carried out (of around a few tens of ms, that is to say a few tens of thousands of “chips”). This rough initial synchronisation is provided by the network, or more precisely by the radio access controller (RNC) controlling several adjacent base stations (also referred to as “B nodes”). A fine synchronisation is then effected regularly by the radio interface between adjacent base stations. The purpose of this fine synchronisation is notably to correct any difference in the sequencing clocks between adjacent base stations. To do this, certain time slots are reserved for the transmission and reception of a synchronisation signal. A time slot dedicated to synchronisation comprises essentially a synchronisation sequence (Sync) and a guard period (GP). Synchronisation is obtained, in a manner known per se, by correlation of the received sequence with a sequence which is a replica of the one transmitted. The correlation is effected on a time window with a length given by the margin of accuracy of the approximate synchronisation. Thus, when a base station receives a synchronisation sequence and detects a correlation peak in this window, it can synchronise its sequencing with that of the adjoining base stations.
The synchronisation sequence generally used is lengthy (a few thousands of “chips”) in order to obtain good accuracy of correlation for an acceptable power per symbol. The guard period must be greater than the propagation time from a base station to an adjacent station so as to avoid, on reception, an encroachment of the synchronisation sequence on an adjacent time slot. The distance between two base stations being greater than the radius of a cell, the guard period (GP) is chosen so as to be greater than the normal guard period (GP′). The guard period (GP) must also take account of the difference between the frame clocks.
The synchronisation sequence is chosen so as to have good autocorrelation properties, namely a very pronounced autocorrelation peak. Generally the synchronisation sequences used are obtained from primitive polynomials on GF(2), a Galois field of cardinal 2. Such a sequence has a length L which is an Nth power of 2 minus 1, that is to say L=2N−1. This is the case notably for so-called Gold sequences proposed in the report TSGR1#15(00)0946 entitled “Sequences for the cell sync burst” of the Working Group TSG-RAN of the ETSI for synchronising adjacent base stations.
Gold sequences have good periodic autocorrelation properties (the correlation of a sequence consisting of the repetition of a Gold sequence with a replica of the sequence of the latter does not have significant secondary peaks). On the other hand, these sequences unfortunately do not have such good aperiodic autocorrelation properties (correlation of an isolated Gold sequence with a replica). What is more, the correlator generally used operates in the time domain in the form of a conventional adapted FIR filter having a complexity in terms of O(L) which can be very high. In addition, the choice of the lengths of such sequences is reduced, since they can, as has been seen, take only values 2N−1 and a truncation would lead to a substantial loss of autocorrelation properties.